1. Field of the Invention
The present invention relates to the fluoridation of inorganic pigment colorants based on rare earth metal sesquisulfides to enhance the coloration thereof.
This invention also relates to novel inorganic pigment colorants, per se, based on rare earth metal sesquisulfides and having improved chromatic characteristics, prepared via the above fluoridation process, as well as to the use of such novel pigments for the coloration of a wide variety of substrates.
2. Description of the Prior Art
It is known to this art that inorganic pigments/colorants are currently widely used for the coloration of a variety of substrates in many industries, especially in those of paints, plastics and ceramics. For such applications, the properties of, inter alia, intrinsic color, coloring power and opacifying power, thermal stability, chemical stability, dispersibility (ability of a product to properly disperse in a given medium) and absence of toxicity, constitute particularly significant criteria to be considered in the selection of a suitable pigment.
The rare earth metal sesquisulfides, of general formula M.sub.2 S.sub.3 in which M represents at least one rare earth metal, are compounds per se well known to this art and described in numerous publications. Moreover, the use of same as pigments/colorants for the coloration of various substrates, such as, for example, plastics, paints and others, is generally described in EP-A-0,203,838, assigned to the assignee hereof.
Additionally, various pigments based on rare earth metal sesquisulfides possessing improved properties (color or thermal and/or chemical stability in particular) are described in FR-91/14,988 and FR-93/04,544, also assigned to the assignee hereof. In said '988 application, for improving the chromatic characteristics of such compounds, pigments based on rare earth metal sesquisulfides are described which have the distinctive feature of additionally containing at least one alkali metal and/or alkaline earth metal element, at least a fraction of which is occluded in the crystal lattice of said sesquisulfides (doped sesquisulfides).
The '544 application, describing improvements at the level of thermal and/or chemical stability, relates to composite inorganic pigments of core/shell type including a substrate based on doped sesquisulfides as indicated above and of a protective layer based on a transparent oxide deposited onto and coating the surface of the substrate.
Nonetheless, need continues to exist in this art for pigments having even more improved characteristics, especially in respect of the chromaticity thereof.